MEMS devices have achieved great attention in the last decade for their board use in many smart systems. The MEMS devices have been utilized for various applications like radio frequency (RF) devices, inertial sensors, optics devices and biomedical systems. In order to strengthen their reliability, these MEMS devices require special protection, such as protection for controlling operating conditions, protection of devices from contamination during dicing, and protection of MEMS fragile hanging structures from harsh environments. Due to the above requirements, MEMS device packaging in great demand is preferred under vacuum conditions or in a controlled atmosphere. Conventional MEMS device packaging uses wafer bonding techniques, which requires a wide dicing area and has low yield during dicing. The MEMS devices packaged thereby have greater thickness due to the wafer bonding techniques, therefore resulting in few devices produced per wafer.
An alternative technique of MEMS packaging/encapsulation to substitute wafer bonding techniques is thin film encapsulation (TFE), as it helps to reduce the thickness and area of a packaged device, as well as the cost of the final device by the elimination of a capping wafer. The encapsulation technique simply uses deposition, etching and release steps of surface micromachining approaches used for MEMS device fabrication. However, conventional encapsulation techniques have drawbacks, such as requiring a long time to release a sacrificial layer and forming mass loading force on the packaged MEMS devices during the sealing process. Conventional TFE techniques have several approaches which differ on the locations of etching channels to remove the sacrificial layer. One of the conventional approaches is to have etching channels all over as well as at the center of the device. In this approach, the sacrificial material can be removed uniformly in a short time. However, this approach suffers from an issue of damaging or mass loading on critical MEMS devices, such as film bulk acoustic resonators (FBARs), as the sealing material will get deposited on the MEMS devices through etch channels that present on the top of the MEMS devices. The other approach is to have etching channels at the side of the MEMS devices. This side etching channel approach may be good at preventing damage from mass loading, but has a disadvantage of a longer release time than regular approaches, as the sacrificial material located deep inside near the MEMS device cannot be efficiently removed through side channels.
Therefore, what is needed is a robust TFE structure and method which solves both the above drawbacks simultaneously, namely which prevents the mass loading on the MEMS devices and enables fast release of the sacrificial layer.